Recovery of oil from oil fields



Dec. 19, 1944. MUSKAT RECOVERY OF OIL FROM OIL FIELDS Filed Oct. 16, 1941 OUTPUT WELL INPUT WELL JVUPr YLS ,of difiering permeabilities.

Patented Dec. 19, 1944 2,365,428 anoovmr or on. mom on. FIELDS Morris Muskat, Oakmont, Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa.,'a corporation of Delaware Application October 16, 1941, Serial No. 415,296

Claims. (01. 166-21) This invention relates to improving recovery of oil from oil fields.

Recovery of petroleum from depleted earth' strata by means of secondary recovery methods has been well known for many years. Two such methods are commonly referred to as gas repressuring and water flooding. More recently it has been proposed to develop virgin oil fields in a systematic manner by spotting output wells and recycling the gas obtained therefrom under pressure to one or more input wells so as to reintroduce it into the producing zones, thus conserving the natural pressure in the formation.

All the foregoing methods present difficulties when the earth formation from which the petroleum is produced is composed of several strata Injected fluids, whether liquid or gas, enter the various producing strata in direct proportion to the relative permeabilities of these strata. As the strata of high permeability are usually more rapidly depleted than those of low permeability, the majority of the extraneous fluid injected into the input well soon passes into the strata which are nearest depletion, if not completely depleted. The stratum of highest permeability eventually becomes substantially entirely depleted and most of the fluid injected into the input wells by-passes the undepleted strata. This is exactly the reverse of what is desired for maximum recovery and maximum economy.

Hitherto it has been proposed to diminish bypassing losses by setting packers in the input .well so as to separate the zones of various permeabilities, after which the extraneous fluid is injected between the packers at pressures and rates appropriate to each individual stratum or zone.

Such a method is only effective when the exact location and boundaries of the several strata are known. Even by the most careful rock coring, or use of electrical logging methods, it is often impossible to locate the depth of beds of different permeability with any certainty. Furthermore, it is usually difficult to set packers and they seldom withstand the pressures employed in most fluid drive methods for any length of time.

The main object of the invention is the provision of a methodof lowering the production ratio of extraneous fluid to oil in recovering oil from subterranean deposits; without requiring accurate placement of packers or other elements at definite levels.

The invention is based on the discovery that a special kind of choke can be provided in input wells in a manner such that in effect it tends to equalize to a remarkable degree, the influx of the, input fluid to loose or by-passlng beds and to tight or oil productive beds. In the simplest embodiment the choke takes the form of an annular body of permeable character, disposed between the well tubing and the several exposed strata faces, and adjusted to have a permeability, of the same order of magnitude as that of the stratum or strata of least permeability, and less than the permeability of the adjacent loose or by-passing beds. The Walls of the choke are made of as great an effective thickness as possible, by enlarging the lower part of the well prior to installing it and by using a small tubing. The net result is a flow-equalizing action which brings about a remarkable improvement in the ratio of oil to extraneous fluid.

In practicing the invention it is important to determina'by core sampling, electrical coring or equivalent means, the permeability of the oil producing sand or sands. But it is not necessary to know the relative depths to or thicknesses of the several strata.

The invention is applicable to both input wells and output wells. For the sake of concreteness it will be described primarily in connection with input wells.

The mode of practicing the invention will be clear from a consideration of the accompanying drawing, which is a schematic view in vertical section of the invention embodied in the input well of a fluid-drive oil recovery system.

The drawing illustrates a typical oil field situation in which a plurality of relatively tight sands, III, II, It! contain oil, which it is desired to recover at an output well I3, with the aid of extraneous drive fluid forced down an input well [4 under pressure. An upper stratum iii of relatively high permeability is associated with less permeable oil-bearing strata. This upper stratum may be one which has never contained oil at all, or may be one which once contained oilbuthas now become exhausted by reason of the more ready flowof oil therethrough under applied drive pressure. Stratum [0, shown as quite permeable, is on the way to becoming such a by-passing stratum, the oil therein being almost exhausted by reason of the relatively high permeability.

On attempting to produce oil in this situation in the ordinary way, the injected drive fluid tends to short circuit through the permeable stratum, leavlng the less permeable oil sands stagnant. It would seem easy to shut off stratum IS in some mechanical way, and thus solve the probfluid input tubing it and the well, throughout the height of the permeable formations into which the injected fluid passes or is desired to pass. This choke can take many forms. It can be a mass of discrete particles, suitably graded fine sand for example; or a mass of special cement which is permeableto fluids.

Advantageously the permeability of the choke is about the same as that of the tightest (least permeable) oil-bearing sand, orat least not varying therefrom by a factor of more than three. If too permeable, the choke lets too much fluid get to the less productive stratum; if too impermeable, needless restriction is imposed on the fluid.

The input tubing it is usually perforated as at it to an extent corresponding to the height of the choke ii, and is fitted with a strainer l9.

In use, the choke imposes relatively little restriction on entry of extraneous fluid to the oil sands of least permeability. The choke, so far as permeability is concerned, can be regarded as a short prolongation of the oil sand, which it resembles closely in permeability. But as regards a highly permeable stratum the choking effect is very considerable, because of the much lower permeability of the choke relative to that of the stratum.

As a. result of these actions, the ratio of extraneous fluid to oil produced and recovered at the output well is markedly improved.

An example will make clear the extent of the improvement obtained. Assuming the drlveefluid admission tubing to be one inch in radius, the radius of the well bore to be 9 inches, the permeability of a by-passing stratum adjacent an oil productive stratum to be 10 times that of the oil productive stratum, and the choke to have a permeability equal to that of the oil stratum, the relative fluid injection capacity of the two strata, per unit thickness, will be lowered from an initial value of 10:1 to 3.06:1. If the high permeability stratum has a permeability of 25 times that of the tighter'formation, a situation not at all infrequently encountered in actual field operations, then the sand or cement choke will change the relative injection capacity .per unit thickness of the loose to the tight stratum from a ratio of 25:1 to 3.54:1, that is, to a value only 14 per cent of the original ratio. At the same time, the absolute value of the injection capacity of the tight stratum will be decreased by only 11 per cent.

As stated, the permeability of the columnar choke need not be fixed with any great precision, since it sumces merely that it be of the same order of magnitude as that of the tightest of the major oil bearing strata. The permeability of the oil-bearing formations is determined by measurement of samples taken from the well; conveniently according to the procedures outlined in my book, The Flow of Homogeneous Fluids Through Porous Media,McGraw-Hill Book Company, New York, 1937, paragraph 2.7 (for a consolidated formation) and paragraph 2.6 (for an unconsolidated formation). When the permegenerating agents ability of the formations is not readily determined, I can proceed empirically. A choke of permeability estimated to be correct is installed and if it is found to be too permeable, it can be removed and a less permeable choke installed. In reparing a choke of predetermined permeability it is often useful to employ sand grains of a range of sizes.

Oil-bearing rocks usually have permeabilities in the range 0.005 to 2 darcies. For'example, in Oklahoma flelds where secondary recovery meth= ods have been employed for several years, some input and production wells are in communication with two strata of permeability 1.2 and 0.05 darcies respectively. In applying the present invention to such wells, the permeability of the choke is between 0.02 and 0.15 darcies.

My method successfully minimizes the flow cf the injection fluids into the loose and depleted zones as compared to the undepleted strata equally well whether the injection fluid be a gas or liquid. In the scope of this invention are included tubular permeable cement chokes wherein the terminal part of the fluid transmitting conduit is entirely eliminated and the conduit simply is attached to a suitably sized hole drilled through and penetrating the cement plug origi nally set to completely fill the hole of the original or under-reamed well bore opposite the producing formation.

It is advantageous to ream out or otherwise enlarge the well, as shown, in the producing region, to as large a diameter as is practicable. On the other hand, the bore of the choke through which fluid is introduced should be as small as convenient.

Placement of the choke is usually simple. Using sand for the choke, the sand can be deposited with the aid of the apparatus shown in Chenault Patent 2,140,072. If a consolidated (monolithic) choke is desired, it is convenient to employ Portland cement containing enough gas (advantageously hydrogen generating agents) to give to the mass the required permeability under the conditions 11 which the mass sets. Such a cement choke is set by methods already known for making permeable cement filter packs.

The invention can be applied with equal advantage to output wells, where it is desired to lessen the proportion of water or gas produced along with the oil.

In practice, the annular opening between well bore it and tubing it is closed off somewhere above choke ii in any conventional manner (not shown), thus limiting flow of fluids in the well to that entering or leaving through tubing it. For this purpose, a packer set anywhere between the well head and the top of the choke it will sufiice, but I often find it more advantageous to run the tubing through a stufilng box at the well head.

What I claim is:

1. In methods of producing oil from subterranean oil reservoirs which include at least one permeable stratum and at least one nearby stratum of higher permeability, wherein a drive fluid is injected into input wells penetrating said formations to drive oil toward spaced output wells penetrating said formations, the improvement which comprises installing in at least one of said wells an elongated tubular choke of a medium permeable in all directions and covering the exposed faces of said strata, the permeability of the medium being substantially. of

the same order of magnitude as that ofthe least permeable stratum and being less than that of said nearby stratum of higher permeability, whereby said choke improves the distribution of extraneous fluid through said strata.

2. A method of producing oil from subterranean oil reservoirs by external fluid drive, such medium being of the same order of magnitude as that of the least permeable stratum and being distribution of flow less than that of the nearby stratum of higher permeability, whereby said choke improves the of extraneous fluid between said strata.

11A method of equalizing influx of input fluids in secondary oil recovery input wells which penetrate formations of different permeabilities and in which oil occurs in at least one formation of relatively low permeability, which comprises establishing over the exposed faces of the formations a permeable choke, the permeability of which is of the same order of magnitude as that of the said relatively low permeability formation.

4. A method according to claim 3 wherein the input fluid is water.

5. A method according to claim 3 wherein the inputfluid is gas.

MORRIS MUSKAT. 

